1. Field of the Invention
The present invention relates to a glow plug and a method for manufacturing the same. More particularly, the invention relates to a glow plug exhibiting excellent durability, being capable of preventing short circuits potentially caused by adhesion of carbon, ensuring safety, and being capable of detecting ion current accurately, as well as to a method for manufacturing the same.
2. Description of the Related Art
In recent years, in order to reduce exhaust gas or exhaust smoke from a gasoline engine or a diesel engine, the engine combustion control system of the engine has been required to detect the state of combustion of the engine. The state of combustion of the engine has been detected in terms of, for example, cylinder pressure, light from combustion, or ion current. Particularly, detection of ion current has been considered useful, since a chemical reaction which accompanies combustion can be observed directly. In order to detect ion current, a glow plug into which an ion detection electrode is incorporated has been proposed (see, for example, Japanese Patent Application Laid-Open (kokai) No. 10-122114).
In the case of a diesel engine equipped with a glow plug into which an ion detection electrode is incorporated, when carbon produced in the combustion chamber adheres to the ion detection electrode, a short circuit is formed, or a leakage current flows, which impairs ion current detection accuracy. Accordingly, the ion detection electrode must be exposed to a region in a temperature zone in which carbon is burned off by a heater. Thus, the exposed portion of the ion electrode is required to exhibit excellent heat resistance and consumption resistance. Conventional glow plugs which have solved the above problems include, for example, a glow plug in which an ion detection electrode is made of a noble metal, such as Pt, in order to ensure heat resistance and consumption resistance thereof, or in which an exposed portion of the ion detection electrode is metallized with a conductive layer (Japanese Patent Application Laid-Open (kokai) No. 10-89687); and a glow plug in which an ion detection electrode is coated with a noble metal, such as Pt, Ir, or Rh, or an insulative porous layer, which is formed by sintering an electrically insulative ceramic powder, such as alumina (Japanese Patent Application Laid-Open (kokai) No. 10-110952 or 10-89226).
However, use of an ion detection electrode or a coating layer made of a noble metal, such as Pt, results in a very expensive glow plug. Also, use of an ion detection electrode made of a noble metal, such as Pt, is likely to cause stress concentration in an insulator in the vicinity of the ion detection electrode. This is because thermal expansion differs between the noble metal and ceramics, which the insulator is made of. As a result, the glow plug may suffer damage, such as cracking. In the case where an exposed portion of an ion detection electrode is metallized with a conductive layer, there is a difficulty in selecting a material for the coating layer. This is because the material must exhibit corrosion resistance at an operating temperature of a glow plug; i.e., 1000.degree. C. or higher, and must be able to prevent separation of the coating layer which potentially results from a difference in thermal expansion. In the case where an exposed portion of an ion detection electrode is coated with an insulative porous layer, the durability of the coating layer may suffer. This is because the porous feature of the coating layer means an increase in the surface area of the coating layer exposed to combustion gas.
Since the tip of a glow plug assumes a high temperature, studies have been carried out on a glow plug in which an ion detection electrode is exposed at a side region of an insulator, not at a tip region of the insulator, so as to ensure heat resistance (see FIG. 1). This configuration involves difficulty in sensing ions which have reached a side region of the insulator opposite the ion detection electrode. Also, the orientation of the ion detection electrode varies depending on the state of attachment of the glow plug, resulting in variations in detection of ion current; i.e., impaired accuracy in detection of ion current.